Do real pearls turn yellow?

Could you elaborate on the potential for genuine pearls to undergo a color change, specifically turning yellow over time? Is this a common occurrence? If so, what factors contribute to this discoloration? Are there any preventive measures one can take to preserve the original hue of their pearls? Furthermore, are there any reliable restoration methods for pearls that have already yellowed, or is this a change that is irreversible? Your insight into this matter would be greatly appreciated.